The invention relates to Czochralski crystal growth apparatus.
The Czochralski (CZ) method is a standard technique for growth of single crystal silicon, gallium arsenide, and related semiconductor materials as well as oxide and sulfide optical and electronic materials.
According to the Czochralski method, a melt is contained in a rotatable, heated crucible and a rotating seed crystal is gradually withdrawn, resulting in the growth of a single crystal of the desired composition. Careful manipulation of temperature gradients at a melt-crystal interface enables growth of single crystalline rather than polycrystalline material.
Key objectives of this crystal growth process are:
(1) production of low defect level crystals; PA0 (2) production of uniformly doped crystals; and PA0 (3) optimization of crystal productivity.
In practice, several problems arise in Czochralski growth. Many of these difficulties are associated with the unsteady nature of the process where melt depletion changes the nature of the circulation system, as well as the heat loss rate.
These systems have been extensively modelled mathematically and there appears to be a consensus that, under ideal operating conditions, forced flow in the melt (due to crystal and crucible rotation) is carefully balanced by natural convective or buoyancy driven flow, produced by heated crucible walls. Further, in this ideal case, forced flow overwhelms natural convection in the bulk.
Another important problem encountered in the operation of Czochralski systems is onset of highly undesirable oscillatory flow behavior, which occurs particularly for large scale systems and manifests itself by the establishment of periodically changing flow patterns.
One way of combatting flow instabilities and oscillatory flow behavior, while promoting uniform dopant distribution, has been imposition of either an axial or a horizontal magnetic field, having strength typically in the few hundred Gauss to the few thousand Gauss range.
Vertical magnetic fields are useful for stabilizing flow; however, they suppress convective heat transport from crucible wall to bulk melt, thereby reducing the crystal production rate by as much as a factor of two.
Horizontal magnetic fields do not interfere with heat transfer from the crucible walls to the melt; however, they may introduce spatial non-uniformities in the dopant distribution .
In conventional CZ systems, including those with magnetic damping, the buoyancy driven flow (in the vicinity of the walls) cannot be controlled independently of the heating rate; furthermore, the buoyancy driven flow varies very markedly as the melt is depleted.